Anchors are well known for mooring vessels at sea. Modern applications have brought new challenges to anchor technology and handling in recent years.
In particular, there is a desire to moor vessels in ever-deeper seas. One industry in which this is particularly important is offshore oil exploration. Although offshore oil platforms sometimes rest directly on the sea bed, this is often impractical. For example, in extremely deep water it is simply not possible to construct an adequate structure of this type, and even where it is possible, it may be prohibitively expensive to do so for speculative drilling projects.
As such, a number of anchored vessels are used for offshore exploration. Semisubmersible platforms are an example of vessels used for this purpose. Semisubmersible platforms have a superstructure that is supported by columns sitting on hulls or pontoons submerged in the water below. Typically, the hulls or pontoons are ballasted with seawater. This design provides excellent stability in rough seas.
A semisubmersible platform supports sufficient machinery and personnel to carry out drilling in deep water, sometimes up to 10,000 meters. There are often 100 or more persons onboard at any given time. These structures are therefore large and the difficulties in ensuring their adequate mooring at such depths are significant.
Semisubmersible platforms are typically supported by eight large anchors, two attached to each corner of the platform by mooring lines. The anchors have fixed flukes and high holding powers. It is not possible for the semisubmersible platform itself to deploy these anchors correctly. This task is instead performed by auxiliary vessels known as anchor handling vessels (AHVs).
AHVs are required both to lay the anchors when the semisubmersible platform is moored and to recover them safely when it is desired to move the platform. Given that a typical anchor used for this purpose may weigh 15,000 kilograms and have dimensions of 8 meters by 7 meters by 6 meters, it is clear that the manipulation of these is not trivial.
In order to control these anchors, AHVs typically use a chaser which consists of a chasing collar surrounding the mooring line and a chaser line extending from the chasing collar. When the anchor is lifted from or lowered into the sea, the chasing collar is disposed around an anchor shackle on the anchor and the chaser line is pulled in or released by the AHV to control the height of the anchor.
A difficulty occurs when it is desired to remove the anchor from the water. Specifically, it is difficult to control the orientation of the anchor as it emerges from the water. Given the size and weight of the anchor, and its design to penetrate surfaces, this risks damage to the machinery used to remove the anchor and the vessels in the vicinity. Moreover, it can also risk damage to the anchor itself.
Control of the anchor's orientation is also important in a number of other anchor handling processes.
International patent application WO 2007/107699 describes an anchor system which is intended to assist in the maintenance of a desired orientation of an anchor. In particular, a chaser stopper is provided in the form of a substantially triangular plate. A chaser has a substantially elliptical aperture for receiving the chaser stopper. As such, if the chaser stopper is pulled tight into the chaser such that the plane of the plate lies along the major axis of the aperture it is not possible for the chaser stopper to rotate. However, this allows the chaser stopper to be retained in two orientations: a desired orientation; and an undesired orientation which is 180 degrees rotated from the desired orientation. Accordingly, the chaser stopper may cause the anchor to become stuck in an undesired orientation.
There exists, therefore, a need to provide assistance for the control of anchors as they are manipulated.